Question

How does `H_3PO_4` behave as an acid?

Answer 1

You have not quoted `K_a` values for `H_3PO_4` or `H_2PO_4^(-)`.......

Explanation:

Phosphoric acid speciates in solution according to......

`H_3PO_4(aq) + H_2O(l) stackrel(K_(a1))rightleftharpoonsH_2PO_4^(-)(aq) + H_3O^(+)`

And.............

`H_2PO_4^(-)(aq) + H_2Ostackrel(K_(a2))rightleftharpoonsHPO_4^(2-)(aq) + H_3O^(+)`

We need `K_(a1)` and `K_(a2)` before we can address your question.